Post combustion CO2 capture with calcium and lithium hydroxide

A small-scale plant was built for measuring the ability of solid sorbents towards the capture of carbon dioxide (CO2) in exhaust flue gas from an internal combustion engine. The investigated sorbents were calcium and lithium hydroxides. Both sorbents are low cost and used in the breathing gas purification systems. The carbonation capacity of each sorbent was measured for different sorbent granulometry (pellets and powder), different temperature (from ambient up to 300 °C), gas space velocity, moisture content and chemical composition of the gaseous stream. The aim was, in fact, to expose the sorbents to a gas stream with chemical and physical parameters close to those at the exhaust of an internal combustion engine. Carbonation capacity was measured with a double technique: on-line by continuously CO2 measurement with a non-dispersive infrared analyzer and off-line by using scanning electron microscopy on carbonated sorbents. Experimental results showed good CO2 uptake capacity of calcium hydroxide at low temperature (between 20 and 150 °C). Performance improvements came from the fine granulometry due to the increased exposed surface area; moreover, the presence of the moisture in gas stream also enhanced CO2 capture. The presence of sulphur dioxide and nitric oxide, instead, greatly decreased the carbonation capacity of sorbents.

www.nature.com/scientificreports/ available in literature are referred to high temperatures (almost 800 °C for absorption and 1000 °C for desorption) 8,9 . The absorption rates are relatively fast and dropped off due to an impermeable build up of carbonates on the surface of solid sorbents. Because of the heterogeneous nature of the reactions, in fact, the formation of a surface product layer of carbonates around the reacting particles at the beginning of a reaction is unavoidable 10 .
In the past, main applications of lithium hydroxide (LiOH) for CO 2 uptake were in space life support systems, space shuttle environmental control and submarine scrubbing systems. The irreversible and exothermic reaction between LiOH and CO 2 occurs at room temperature with a high absorption capacity because of the low molar mass of LiOH. The adsorption capacity of LiOH is strongly dependent from moisture content in the CO 2 stream 11 . A study on LiOH adsorbed zeolytes demonstrated that at ambient temperature the maximum CO 2 uptake occurred at 65-70% relative humidity values 12 . Formation of lithium hydroxide monohydrate as an intermediate compound has been postulated to explain the effect of water vapor on the reaction 13 . For this reason, it can be easily exploited for CO 2 removal from moisture-rich exhaust gas of an internal combustion engine.
The two solid sorbents were examined in a laboratory pilot plant which simulates a CO 2 absorber unit for a flue gas of an internal combustion engine. Sorbents are arranged in a fixed bed reactor and several operation parameters (space velocity, granulometry, temperature, moisture content) were investigated.

Materials and methods
Solid sorbents. CO 2 capture was studied with two low cost solid sorbents: soda lime and lithium hydroxide.
Among the current uses of both hydroxides, there is the breathing gas purification systems for medical devices, spacecraft, submarines and rebreathers to remove carbon dioxide from exhaled gas. Soda lime (Medisorb by GE Healthcare) is composed by 75%w Ca(OH) 2 , 3%w NaOH and water. It appeared as white pellets (mesh 2.5-5 mm) classified as irritant for eyes, skin and respiratory system (Table 1). It has a relative density of 2 g/ cm 3 and is slightly soluble in water. Pellets were also grinded to obtain a fine size of soda lime (powder, mesh 10-50 μm) to be exposed to CO 2 uptake.
Two formulations of lithium hydroxide were tested: anidrous (LiOH) and monohydrate (LiOH•H 2 O). LiOH was provided in pellets and LiOH•H 2 O in powder, mesh 30-250 μm (Sigma-Aldrich, reagent grade ≥ 98%, Table 1). Their classification indicates acute toxicity and skin corrosion. Relative density of LiOH is 2.54 g/ cm 3 , and that of LiOH•H 2 O is 1.51 g/cm 3 . LiOH anidrous and monohydrate were tested separately and mixed (50-50%v). The mixed solid is characterized by a mixed granulometry (powder and pellets) which has two main advantages. The first is the greater surface area compared with anidrous pellets, which obviously enhances the CO 2 uptake; the second is a lower water content compared to hydrate solid, which avoids the undesired effect of grain agglomeration. Figure 1 reports SEM image of both sorbents.
Pilot-plant description. Figure 2 schematizes the laboratory plant developed for the experimental activity. A quartz cylindrical reactor (length = 50 cm, diameter = 1.4 cm) was used for testing the solid sorbents. The sorbents were inserted inside the reactor and fixed by using fiberglass tips. Although gas flow was set at 8 l/min, space velocity (volumetric flow per volume unit, a key parameter for the after-treatment catalyst design) was changed by varying the height of sorbent inside the reactor.
The reactor was housed in a cylindrical heater able to set the temperature from the ambient up to 1000 °C. A thermocouple is inserted into the furnace to control the temperature reached near the reactor. The inlet flow to the reactor was regulated by a Mass Flow Controller (0-20 l/ min, Bronkhorst), connected to the gas bottle containing CO 2 ; the output flow of the reactor was sent to a moisture separator, a flow meter and then to the Non-Dispersive Infra-Red (NDIR) analyzer by Horiba. On the sampling line of the analyzer, a vent was inserted for the overflow exiting the reactor. The valves V3 and V4 were inserted to allow the measurement of the gas inlet concentration. In this case, in fact, the gas bottle is directly connected with the analyzer, without going through the reactor.
In order to analyze the effect of water content, the inlet gas, before to reach the reactor, was diverted by a three-way valve (V2) to the humidifier (Permapure's NafionTM MH-Series Humidifier tube). In this way, the sorbent was studied in dry and wet conditions. The distilled water for the humidifier is contained in a graduated syringe and a thermocouple records its temperature. At the humidifier outlet, the wet stream to be treated was sent to the reactor.
The water content entering the reactor is measured by the Horiba analyzer using NDIR (Non-Dispersive InfraRed Detector) detectors. Furthermore, the total volume of water absorbed by the system during the whole   www.nature.com/scientificreports/ Carbon uptake of sorbents was also evaluated off line throughout scanning electron microscope (SEM Phenom Pro X) analysis of exposed hydroxides. The SEM is equipped with an Energy Dispersive Spectrometry (EDS) detector for elemental analysis. Low acceleration voltage of 5 kV was used for imaging in order to prevent back scattering phenomena 14 . Voltage of 15 kV was, instead, used for EDS analysis. For SEM-EDS analysis, not exposed samples were compacted into tablets whereas exposed samples were fixed on the aluminum holder throughout a carbon sticker.

Results and discussion
Calcium and Lithium hydroxide performance. CO 2 uptake of Ca(OH) 2 was tested in the range between ambient temperature and 300 °C. Results are reported in Fig. 3 which shows carbonation capacity (molCO 2 /kg sorbent) in dry conditions for both pellets and powder. Ca(OH) 2 pellets were tested for two space velocity (SV): 31,200 and 15,600 h −1 . Looking at all data, the highest carbonation efficiency is observed in the range of temperatures from 80 to 100 °C. For temperature higher than 100 °C, the carbonation capacity decreases because of the loss of sorbent humidity. As mentioned above, in fact, soda lime contains almost 10-15% of water which promotes the carbonation reaction. Indeed, where the temperature makes possible the water evaporation (> 100 °C), the carbonation reaction is not enhanced.
At the investigated temperatures, main carbonation reaction involves directly Ca(OH) 2 according following Eq. (2).
It takes place until 350-400 °C; at higher temperatures, instead, dehydration of Ca(OH) 2 to CaO can occur 15 . Some papers indicate that the optimal temperature of Ca(OH) 2 carbonation is almost at 200 °C 16 . Carbonation capacity trend as a function of the temperature, reported in Fig. 3, confirms this statement.
The carbonation capacity of calcium hydroxide is dependent on Space Velocity. The variation of this parameter was obtained by varying the height of the reactor filling (10 and 20 cm for 15,600 and 31,200 h −1 , respectively). By grouping the data with the same space velocity and same granulometry (pellets), it should be stated that an increase in space velocity determines an average reduction in carbonation capacity of almost 25% (Fig. 4a). Therefore, a longer contact time favors the CO 2 capture capacity.   www.nature.com/scientificreports/ For stating the influence of sorbent granulometry, data at space velocity of 31,200 h −1 were grouped according sorbent granulometry, pellets and dust (Fig. 4b). Fine granulometry involves an increment of carbonation capacity of almost 40% compared to pellets. Main reason is the increased surface area of dust which is involved in the carbonation reactions. It has to be noted that the fine granulometry corresponds to the best absolute result obtained with soda lime (almost 3.6 molCO 2 /kg sorbent corresponding to 35% of maximum possible carbon uptake by the tested sorbent mass). Figure 5 shows carbonation capacity measured with LiOH anydrous and monohydrate, as a function of the temperature. Data are referred to a space velocity of 32,100 h −1 . When increasing temperature, carbonation capacity of LiOH decreases: the highest capacity of 4 molCO 2 /kg sorbent is measured at the ambient temperature. At T = 120 °C, the carbonation capacity drops to 1.5 mol CO 2 /kg sorbent. The increasing of temperature, in fact, does not promote the formation of monohydrate hydroxide and then carbonation rate decreases 17 .
Main reactions (Eqs. 2 and 3) occurring for CO 2 absorption involve the production of the intermediate hydroxide monohydrate; then, it reacts with CO 2 to form lithium carbonate.
Therefore, the efficiency of CO 2 absorption is strictly influenced by water adsorption rate on the LiOH surface. As consequence, the percentage carbonation capacity measured for anydrous LiOH is almost 13% of maximum CO 2 mass which is possible to capture, by considering the stoichiometry. Moreover, it was observed that, despite the presence of water, the carbonation capacity of LiOH monohydrate is always lower than 0.5 mol CO 2 /kg sorbent. This is caused by the too fine granulometry of the available sorbent; in fact, since CO 2 absorption reactions lead to the water formation in addiction to that already present in the LiOH⋅H 2 O, the granules agglomerate forming macro-granules characterized by a reduced surface area. Better performance than the two pure sorbents were obtained by mixing the two hydroxides (50% v/50% v). In this way the new mixed sorbent was characterized by a mixed granulometry (pellets and powder) and also by a reduced water content compared with the pure lithium monohydrate hydroxide. The carbonation capacity of mixed sorbent is greater than 4 mol CO 2 / kg sorbent, reaching the best performance with lithium sorbent (26% of CO 2 captured compared with the maximum allowed).
SEM/EDS analysis. Some samples of soda lime exposed to CO 2 gas stream were also analyzed with SEM to obtain elemental composition. Figure 6 shows the elemental composition of soda lime exposed to a gaseous mixture containing CO 2 at 20%v in nitrogen, at the reactor temperature of 120 °C and with a Space Velocity of 31,200 h −1 . The graph shows the compositions for the fresh (not exposed) sample and for 2 carbonated samples: original particle size (pellets) and fine particle size (powder). The data correspond to 20 scans average for each sample analyzed. SEM analysis confirms that the fine grain size shows a greater carbonation capacity. In fact, the average percentage concentration of C measured in the exposed soda lime powders is 21%; the soda lime of original grain size, instead, has a C content of approximately 16,7%.
Considering that the fresh sample has an initial carbon content of 13,6%, the C trapped is 7,3% for the powders and 3,1% for the pellets. Starting from these percentages, the carbonation capacities were estimated in the two samples of soda lime, considering that the number of moles of CO 2 trapped by the sorbent coincides with the number of carbon atoms. The results are compared with carbonation capacity estimated by online CO 2 measurement downstream of the reactor (left side of Fig. 6). The greater exposure of the fine particle size to the gaseous stream allows to obtain a good agreement between the direct measurement of the carbonation capacity and the indirect measurement by means of elemental SEM/EDS analysis (difference of 4%). When analyzing the pellets, the difference is greater (+ 40% of the direct measurement compared to the SEM).  . It was demonstrated that the reaction mechanism is dependent by alkalinity of adsorbed water on Ca(OH) 2 surface 22 . Equations from (4) to (9) describe the possible reactions in presence of water.
For higher water alkalinity, carbonate ion is predominant and carbonation reaction (9) is favored; otherwise, bicarbonate ion is predominant (reactions (6) and (8)) and dissolved in water layer. Reactions of adsorption and hydration of CO 2 and the formation of carbonate ion are very fast, whereas the dissolution of Ca(OH) 2 may be slow, depending on the adsorbed humidity.
In order to study the effect of moisture on CO 2 capture, wet conditions were realized in small-scale reactor by using a NafionTM tube humidifier, positioned upstream of the reactor inlet. In particular, it was possible to achieve 2 and 5% as humidity (H). It has to be noted that the addition of moisture in gas stream adds also the advantage to simulate chemical gas composition closer to those of an internal combustion engine exhaust (water concentration between 5-10% v).
Each condition was examined with the Ca(OH) 2 sorbent both in pellet and in powder, in the temperature range between ambient temperature and 150 °C. The Space Velocity used for these tests was 31,200 h −1 . Figure 7 reports data of wet and dry carbonation capacity as a function of temperature. By increasing the water content, an improvement of carbonation capacity is clearly visible only for pellets. Soda lime powder shows a higher CO 2 uptake only in correspondence of few temperatures (80 and 120 °C).
In order to deeply investigate the influence of humidity, the box plot of Fig. 8 summarizes the average carbonation capacity measured in dry and wet conditions (H = 2% v and 5% v). The data were grouped according to the granulometry of the sorbent. It is evident that in the investigated experimental conditions, the humidification of the gas stream introduced appreciable benefits for the pellets. In this case, in fact, the average carbonation capacity goes from 2,3 molCO 2 / kg sorbent to 3.9 molCO 2 / kg sorbent. For the powder size, on the other hand, the improvement in the carbonation capacity for the humidification of the gaseous current is low due to agglomeration phenomena of the sorbent with a consequent decrease in the surface area.
These results are comparable with literature. Recently 23 , investigated the CO 2 capture by using a fluidized bed of calcium hydroxide, previously humidified. They analyze the capacity of CO 2 uptake at ambient temperature, atmospheric pressure and 1%v CO 2 inlet concentration. When increasing the relative humidity from 24 to 100%, carbonation capacity doubled up to almost 0,3 molCO 2 /kg sorbent. A similar research carried out by 24 , showed  Influence of chemical composition of gas stream on CO 2 capture. The interference of other gas on CO 2 capture was studied. The attention was focused on some compounds normally present in the exhaust flue gas of internal combustion engine. To this aim, carbonation capacity of soda lime was monitored by using the following gas mixtures: Mix 1: CO 2 (10-20%v) in nitrogen Mix 2: CO 2 (10%v), CO (0,5%v), C 3 H 8 (330 ppmv), NO (1000 ppmv) in nitrogen Mix 3: CO 2 (6%v), SO 2 (400 ppmv) in nitrogen Mix 2 has a chemical composition close to that of the engine exhaust, whereas mix 3 was used to analyze the interference of sulphur on the carbonation capacity of soda lime. Figure 9 reports average carbonation capacity of Ca(OH) 2 as a function of chemical composition of gas mixture. Data are grouped according sorbent granulometry (pellet or powder). It should be noted a lower CO 2 uptake in presence of other gaseous compounds compared with the binary mixture of CO 2 and nitrogen. Compounds such as NO, CO, C 3 H 8 and SO 2 reduce carbonation capacity more than 60%. SO 2 interference was deeply investigated varying the reactor temperature and moisture content in gas stream (Fig. 10). Each graph reports carbonation capacity with and without SO 2 as a function of the temperature. Moisture content and granulometry are fixed.
The efficiency reduction in CO 2 capture increases as the humidity of the gas stream increases. In dry conditions, differences in the carbonation capacity due to the presence of SO 2 is almost 15% for pellets and 29% for powder. The greatest difference in carbonation capacity is, however, measured at a water vapor concentration of 5%v. In this case reduction of carbonation capacity drops of almost 50% compared with data measured in absence of SO 2 (data available only for Ca(OH) 2 in pellets). Many literature studies have shown that the carbonation efficiency with calcium-based sorbents is significantly reduced by the presence of SO 2 , due to the irreversible reaction between Ca and SO 2 to form CaSO 4 25,26 . Furthermore, the sulphate forms a surface layer on the sorbent particles and prevents the diffusion of CO 2 through the pores of the sorbent itself. www.nature.com/scientificreports/ Conclusions CO 2 capture from solid sorbents seems to be an interesting technique to control CO 2 emissions from internal combustion engine. The paper investigated the performance of low cost, not toxic and safe sorbents.
Main results show that soda lime (mainly calcium hydroxide) was found to be a good sorbent to be used for capturing CO 2 from effluents at low temperatures (between 20 and 150 °C).
The parameters that significantly influence the carbonation capacity are: • grain size: fine grain size has a greater surface area available for carbonation reactions;   www.nature.com/scientificreports/ • Humidity: increasing the humidity of the current to be treated increases the carbonation capacity; • Presence of other compounds: the presence of other compounds such as SO 2 and NO significantly reduces the CO 2 capture efficiency The use of fine grain size sorbents presents the disadvantage of agglomeration phenomena of the sorbent itself, especially in humid conditions. The negative effect found on the carbonation capacity from the presence of SO 2 , as this can compete with CO 2 in the absorption, leads us to say that a CO 2 capture system of this type must be installed downstream of a removal system of sulfur oxides (and preferably also nitrogen oxides), such as a scrubber or an SCR system.
Carbonation capacities of the order of 5 mol of CO 2 per kg of sorbent have been obtained. The use of an inert porous substrate could significantly improve the performance of hydroxides in terms of CO2 capture efficiency, since it could avoid blockages due to sorbent agglomeration phenomena. Even if it involves an increase in the weight of the overall sorbent.